This invention relates in general to gas detectors and in particular to a new and useful optical multi-beam gas measuring device.
The specific purpose of a measuring apparatus in accordance with the present invention is to measure gases in which extraneous gases absorbing in the same wavelength region are admixed. For example, the alcohol content in exhaled air can very accurately be measured through infrared absorption of a wavelength of 3.4 microns. However, acetone traces, which may be present in the exhaled air due to sickness or excessive fasting, for example, may be mistaken at this wavelength for alcohol which is not present. Acetone may be detected by measuring the optical transmission at 3.25 microns, which wavelength is virtually no longer absorbed by alcohol.
In a prior art apparatus for measuring the proportion of a component of a mixture transmitting radiation, the beam is directed by optical means from a source of optical radiation through a gas vessel containing the mixture to be checked, to a following beam splitter by which the beam is split into two separate, parallel partial beams. Further, filters for selecting various wavelengths are provided in the paths of each partial beam, aside from other auxiliary means. Behind the filters, a collective lens for concentrating the partial beams is followed by a detector and a device for processing and evaluating the signal. A chopper is provided for alternately directing the signals of the partial beams to the processing device.
The beam splitter is embodied by a prism system comprising a rectangular central prism whose right-angle edge pointing against the radiation is positioned symmetrically in the radiation axis. The prism splits the beam by reflection at its surfaces into two partial beams which are at right angles to the axis of the original beam. Aside from the central prism, two equalsided right-angle lateral prisms are so positioned that upon entering a lateral prism, each partial beam is reflected from the hypotenuse face thereof at 90.degree. to emerge parallel to the axis of the original beam. The manufacture of the central and lateral prisms as single-piece bodies as well as their mounting and adjustment are very expensive. Further, the prisms are highly sensitive to stresses. A re-equipment of the apparatus with partial beam filters suitable for other gases is also very expensive. (German AS No. 23 50 004).